1. Field of the Invention
The invention concerns novel fast-drying pressuresensitive adhesives having excellent cohesive strength and improved peel resistance. The invention also concerns pressure-sensitive adhesive tapes which usually are slit to narrow widths and then wound upon themselves for storage and shipment. Such tapes are originally produced in wide widths, and the term "tape" encompasses such wide widths. The invention specifically concerns removable pressure-sensitive adhesive tapes such as masking tapes, weatherstripping tapes, label tapes, and protective plating tapes.
2. Description of the Related Art
As is pointed out in U.S. Pat. No. 4,629,663 (Brown et al.), the manufacture of pressuresensitive adhesive tapes is being shifted from organic solvent systems to aqueous latices in order to reduce air pollution and fire hazard and also to eliminate the rising expense of organic solvents. Surfactants, wetting agents, or emulsifying agents are commonly used to protect the aqueous latex from coagulation. Among patents cited in the Brown patent to illustrate this is U.S. Pat. No. 3,770,708 (Knoepfel et al.) which discloses a pressure-sensitive adhesive tape, the adhesive layer of which is a copolymer of vinyl monomers such as a non-tertiary alkyl acrylate and zwitterionic monomer which in part serves as an emulsifying agent that is bound to the polymer, such an emulsifying agent being called an "emulsifier monomer". All of the Knoepfel working examples also employ conventional emulsifying agents which do not become bound to the polymer and are called "external emulsifiers".
The pressure-sensitive adhesive tape of U.S. Pat. No. 3,922,464 (Silver et al.) is similar to that of the Knoepfel patent except in these respects: (1) it is removable, (2) from 0.2 to 5% by weight of at least one vinyl-unsaturated, homopolymerizable emulsifier monomer is copolymerized with the Knoepfel monomers, and (3) the zwitterionic monomer can be omitted. The Silver adhesives also preferably employ "small amounts of external emulsifier, especially where short chain emulsifier monomers are used. Such emulsifiers, which are commonly employed in the preparation of many copolymer latices, assist in initial suspension of the monomeric reactants and tend to result in a more homogeneous appearing emulsion than when they are omitted. On the other hand, excessive amounts of such external emulsifiers tend to ... make the emulsion more difficult to dry" (col. 8, lines 39-55).
Both the Silver patent and the above-cited Brown patent specifically concern pressure-sensitive adhesive tapes that are removable for uses such as automotive masking which require clean removability after baking, e.g., of automotive paint for one hour at 120.degree. C. The pressure-sensitive adhesive tape of the Brown patent, approximately like that of Silver, is a copolymer of monomers comprising
(a) from 95 to 99.8 parts by weight of at least one terminally unsaturated vinyl monomer, 60 to 100 weight percent of said vinyl monomer being selected from the class of nontertiary alkyl acrylates wherein each alkyl group has at least half of its carbon atoms in a single chain and the average length of the alkyl chain is at least 4 and not more than 12, and PA0 (b) from 0.2 to 5 parts by weight of at least one vinyl-unsaturated, homopolymerizable emulsifier monomer which is a surfactant having both a hydrophobic and a hydrophilic moiety, contains at least 5 but not more than 40 carbon atoms, and is water-dispersible, PA0 (a) from 95 to 99.9 parts of at least one terminally unsaturated vinyl monomer, 60 to 100 percent of said vinyl monomer being selected from the class of nontertiary alkyl acrylates wherein each alkyl group has at least half of its carbon atoms in a single chain and the average length of the alkyl chain is at least 4 and not more than 12, and PA0 (b) from 0.1 to 5 parts by weight of at least one vinyl-unsaturated homopolymerizable emulsifier monomer which is a surfactant having both a hydrophobic and a hydrophilic moiety, contains at least 5 but not more than 40 carbon atoms, and is water-dispersible, PA0 A = sodium dodecyl benzene sulfonate ("Siponate" DS10, Alcolak, Inc.) PA0 B = sodium alkylaryl polyethoxy sulfonate ("Triton" X-200, Rohm & Haas) PA0 C = monosodium-n-lauryl-.beta.-iminodipropionic acid ("Deriphat" 160 C, Henkel) PA0 D = potassium alkylaryl polyethoxy phosphate ("Triton" QS-44K, Rohm & Haas) PA0 E = potassium alkylaryl polyethoxy phosphate ("Crafol" AP50, Pulcra S.A.) PA0 F = potassium alkylaryl polyethoxy phosphate ("Servoxyl" VPNZ 10/100, Servo) PA0 G = potassium alkylethoxy phosphate ("Crafol" AP60, Pulcra SA) PA0 H = potassium alkyl phosphate ("Servoxyl" VPTZ, Servo) PA0 I = potassium alkyl polyethoxy phosphate ("EAK" 8190, Rewo Chemical Group) PA0 J = potassium alkyl phosphate, ("Berol" 522, Berol Kemi A.B.)
the total parts by weight of (a) plus (b) being 100, and said adhesive layer preferably contains a small amount of an external emulsifier.
The Brown patent says that its adhesive "differs from that of the Silver patent in that the emulsifier monomer comprises a monovalent salt of a styrene sulfonate. As compared to emulsifier monomers named in the Silver patent, a monovalent salt of styrene sulfonate more reliably produces coagulum-free latices, both the latices and polymers produced therefrom tend to have greater hydrolytic stability, and latices of higher solids can be obtained which are easier to coat" (Col. 3, lines 16-24).
U.S. Pat. No. 4,645,711 (Winslow et al.) concerns what was "believed to be the first nonstaining automotive masking tape that reliably resists typical lifting forces at 150.degree. C. from automotive paint and yet can reliably be stripped off cleanly from automotive paint after one hour at 150.degree. C. ... By `typical lifting forces` are meant those simulated in the `Curved Panel Lifting Test at 150.degree. `" (col. 3, lines 3-10) described both in the Winslow patent and hereinbelow. The Winslow tape uses the same copolymer as in the Silver patent but differs "in that its layer of pressure-sensitive adhesive comprises from 5 to 50% by weight of" at least one of four classes of tackifier resins. After noting the Silver patent teaching that small amounts of external emulsifiers assist in initial suspension of the monomeric reactants, the Winslow patent prefers using at least 0.05 weight percent of external emulsifier for the same reason (col. 4, lines 31-42).
The backing of each pressure-sensitive adhesive tape of the working examples of the Brown and the Winslow patents is polyester film. When a paper backing has been substituted for the polyester film and the amount of the emulsifier monomer has been within the preferred range of the Brown patent (0.5 to 2 weight percent), it has been necessary to use very low heat in drying the adhesive coating to avoid bubbling of the adhesive layer. This slows the coating process to the point that it is not commercially feasible to use a paper backing for making the preferred tapes of either the Brown or the Winslow patent. While the adhesive coating could be dried more quickly at reduced amounts of the emulsifier monomer, such reduced amounts would result in undesirably reduced cohesive strengths.